A liquid crystal display (LCD) includes a liquid crystal alignment film. The liquid crystal alignment film is primarily made of polymer materials. The liquid crystal alignment film directs the alignment of liquid crystal molecules. When the liquid crystal molecules are moved by the influence of an electric field to display an image, the liquid crystal alignment film allows the liquid crystal molecules to be oriented in a predetermined direction. Generally, it is necessary to uniformly align the liquid crystal molecules in order to provide uniform brightness and a high contrast ratio to the liquid crystal device.
The conventional method of aligning the liquid crystal includes coating a polymer membrane such as a polyimide on a substrate made of a material such as glass, and rubbing the surface of the substrate with a fiber such as nylon or polyester in a certain direction. However, the rubbing method may cause serious problems when fabricating a liquid crystal panel due to fine dust or electrostatic discharge (ESD) that may be generated while rubbing the polymer membrane with the fiber.
In order to solve the problems of the rubbing method, a photo-radiation method has recently been researched to induce anisotropy to the polymer membrane by irradiating light on the membrane so as to align the liquid crystal molecules.
Polymer membrane materials for the photo-alignment method can include polymers having photo-functional groups such as azobenzene, coumarine, chalcone, and cinnamate. Such polymers are anisotropically photo-isomerized or photo-cross-linked by being irradiated with polarized light to provide anisotropy to the surface so that the polymer can induce the liquid crystal molecules to align in a certain direction.
The material for the liquid crystal alignment film should have optical stability and thermal stability, as well as no after-image, in order for it to be useful in a substantial liquid crystal display device (LCD). However, the conventional photo-alignment materials have many problems in this respect.
Further, conventional materials used for liquid crystal photo-alignment films are mainly polymeric, that is, have a polymer main chain and a photo-functional side group that is capable of inducing the photo-anisotropy, such as azobenzene or cinnamate. When such materials for a polymeric liquid crystal photo-alignment film are used, the materials may require a lot of photo-energy to induce the anisotropy, and the thermal stability, optical stability, and electro-optical characteristics thereof can be significantly affected by any remaining or residual unreacted photo-functional groups.
In general, a liquid crystal display can be fabricated by coating a liquid crystal photo-alignment agent on a glass substrate deposited with a transparent indium tin oxide (ITO) conductive layer and heating it to form a liquid crystal alignment film, and then combining two substrates oppositely facing each other and implanting the liquid crystals therebetween. Alternatively, a liquid crystal display can be fabricated by dripping liquid crystals on one substrate and combining it with another substrate oppositely facing the one substrate. In particular, fifth generation, or later, liquid crystal displays used in medium- and large-sized product lines are typically produced using the latter method.
In general, a liquid crystal photo-alignment film can be formed by coating a liquid crystal photo-alignment agent prepared by dissolving polyamic acid or polyimide in an organic solvent on a substrate in a flexo printing method, and then predrying and firing it. When the liquid crystal photo-alignment agent has poor printability, it may have a film thickness deviation, and accordingly may negatively influence display characteristics of a liquid crystal display including the film.